The application of electrical stimulus to neuromuscular structure for beneficial purposes is well known. Treatment of this type is employed for pain and edema reduction, neuromuscular re-education, and reduction of spasticity.
The electrical and biochemical aspects of muscle contraction are relatively well known, and are described in detail by Benton et al. "Functional Electrical Stimulation--A Practical Clinical Guide", second edition, Ranchos Los Amigos Rehabilitation Engineering Center, Downey, Calif., which is incorporated herein by reference. Electrical muscle stimulation (EMS) has significantly enhanced muscle function in individuals suffering from neurological impairment due to stroke, cerebal palsy, and other conditions which effect the muscular system.
Muscle contraction is a complex electrical-biochemical event which requires transmission of an electrical signal from the brain to the localized nerves within the muscles. If the signal has sufficient amplitude and frequency, a biochemical reaction is established within the nerve fibers generating a series of constructively interfering muscle twitches culminating in a strong contraction. A signal which is inadequate either in electrical amplitude, or frequency does not result in a summating contraction known as "tetanization".
In individuals having healthy nervous systems, electrochemical signals of between 800 to 1000 .mu.V can be generated during a strong muscle contraction. In individuals having impaired neuromuscular systems, the initiating signal for strong muscle contraction can be as small as 100 to 250 .mu.V, substantially below the threshold value necessary for tetanization. These signals can be received and viewed on an electromyogram. This electromyographic (EMG) signal is a complex sinusoid having a frequency of approximately one kH.sub.z. This signal can be used to trigger an externally generated therapeutic signal causing the desired muscle to contract when a patient undergoing therapy generates the EMG signal. This is described on pages 17 and 41 by Benton et al. The relationship between applied voltage and time, or current strength and time, respectively is well known for applications employing external electrodes on the surface of the skin. To excite impaired muscle, a signal having a magnitude of 90 V for 10 msec, or 50 V for 300 msec is sufficient. A strong wrist extension can be achieved by applying a current amplitude of 60 mA for 100 .mu.sec or 40 mA for a duration of 300 .mu.sec.
It is now well known that a variety of additional factors influence the therapeutic effect, patient comfort, and safety of EMS therapy. It is well known for example that muscles cannot be continuously stimulated and that an appropriate duty cycle (ratio of time on to time off) of 1:3, to 1:5 is desired. EMS signals are also preferably applied in pulses varying in polarity (i.e. bipolar) with respect to the background electromyographic signal of a patient at rest. Furthermore, the EMS signals should be delivered in a preferred frequency range of 30 to 70 pulses per second. Further yet, patients are most comfortable if the initial pulses are at less full amplitude, and slowly grow to the fully desired amplitude. Still further, pulses of differing shapes (i.e. square wave, exponentially decaying trailing edge, etc) each have their own beneficial purpose. Moreover, the treatment should not apply net direct current to the patient. Thus, even if positive and negative pulses have different wave forms, the areas under each pulse should be equal. Superposition of the above parameters can result in a pulse train 12 in FIG. 2 consisting of a series of individual pulses 14 having varying magnitude so as to establish a leading edge slope or "rise time" 16, a general magnitude of 18, a pulse frequency defined by the pulse period 20, and a total duration defined by the length (in the time domain) 24 of the pulse train 12. As previously stated, the width 24 of each pulse can also vary for different therapeutic effects.
As shown in FIG. 2, a positive pulse in the form of a square wave may be combined with a negative pulse in the form of an exponentially decaying function 26. Enlarging the pulse width 24 requires an increase in the time constant of the exponential function 26 so that the areas under each curve is equal and a DC charge is not imparted to the patient. The assignee of the present invention has also discovered that the fall time 28 of the pulse train envelope is also significant for patient comfort.
It should be appreciated that the therapist operating equipment which generate EMS signals as shown in FIG. 2 is now confronted with a difficult and tedious task in setting up and monitoring the EMS equipment. Furthermore, most prior art devices use electronic circuits employing discrete analogue, and simple digital circuits such as those shown in FIG. 3 which do not facilitate modifications of the parameters discussed above by the therapist. For example, the neuromuscular stimulating apparatus disclosed by Alon, U.S. Pat. No. 4,690,146 employs discrete ramp generators, transition detectors, On-Off timers and the like to control some of the parameters described above. However, the majority of the parameters which make up the pulse trains (the stimulation signal) are not variable. Control of these parameters by the therapist is either unavailable or is by a means of dials and switches. The Dynamic Servostim.TM. electromyographic triggered, electrical stimulator (Model 1023) manufactured by Electronic Medical Instruments, Inc., Bellevue, Wash. is typical of the state of the art for multichannel (i.e. capable of stimulating more than one muscle group) systems which provide a rectangular monophasic pulse train, variable amplitude, adjustable pulse rate, and pulse width selectable at either 0.3 msec or 1.0 msec. The initial amplitude of the pulses rises to full amplitude within 0.1 seconds to maximize sharpness of the proprioceptive feedback wave front and is not adjustable. Readouts of these parameters are provided by discrete LEDs, and an LED numeric display. When using both channels to energize complimentary muscle groups in a rhythmic fashion, the set up procedure can be complex and time consuming.
Various prior art devices have attempted to control set up parameters such as: pulse amplitude, pulse duration, pulse recurrence frequency, duration of a pulse train, the time interval separating two consecutive pulse trains, and the slope of the pulse trains under microprocessor control as in U.S. Pat. No. 4,919,139 to Brodard. However, the shape of individual pulses are not controllable and there is no significant enhancement to the therapist's task in establishing the parameters, although they are recorded on EEPROM cards associated with individual patients. Still other devices which employ personal computers, such as Maher et al., U.S. Pat. No. 4,832,033 facilitate preprogramming of individual simulation devices but do not significantly reduce the therapist's tedious and complex task of establishing and monitoring complex stimulation signals prepared for each patient individually.
Therefore, a need exists for an electrical muscle stimulation apparatus which permits instantaneous control of all the applicable parameters which define a stimulation signal, and which assists the therapist in constructing and monitoring the application of such signals.